Comparison of the effects of potassium and membrane potential on the calcium‐dependent sodium efflux in squid axons.

Abstract

SUMMARY1. Experiments are described in which the [Ca]o-dependent component of 22Na efflux is monitored under conditions of membrane potential control by voltage clamp.2. The apparent affinity of the efflux system for external Ca is very low in choline sea water (apparent KD 50 mM); but increases dramatically when choline is replaced isosmotically by Li or K (apparent KD -1-2 mM). Ca influx changes in a parallel fashion. Tris behaves much like choline and guanidinium is about two-thirds as effective as Li.3. Replacement of Li by K has little effect on the apparent affinity for external Ca but brings about a small (30-40 %) increase in the maximal flux. The increase in maximum flux can be removed by electrical hyperpolarization to the potential before application ofK and, in the absence ofK, can be mimicked by electrical depolarization. These experiments suggest that the stimulatory effect of K on the Ca-dependent Na efflux into Li sea water is electrical in origin.4. Partial replacement of choline by K stimulates the Ca-dependent Na efflux; but only part of this stimulation can be removed by electrical hyperpolarization and, in the absence of K, electrical depolarization only brings about a relatively small stimulation. This is because only part of the stimulation that follows addition of K to choline sea waters is electrical in origin: the rest reflects an increase in the apparent affinity for external Ca that is brought about by K acting chemically. The maximum efflux into K is about 40 % higher than that into choline. That this may reflect an electrical effect is supported by the observation that electrical depolarization increases the flux into choline sea water containing 110 mm-Ca where the Ca-binding site is close to saturation.5. The voltage clamp was used to determine the voltage dependence of the Ca-dependent Na efflux into Li sea water, choline sea water and choline sea water containing 100 mM-Na. In all three cases the flux increased with depolarization and was still rising at + 70 mV. The dependence on potential was not very steep, an e-fold increase occurred over approximately 50 mV.

Influence of membrane potential on calcium efflux from giant axons of Loligo.

“…The methods used were the same as described in the previous paper (Allen & Baker, 1986), with the following modifications: 46Ca was concentrated such that microinjection introduced into the axon about 106 counts/min. This raised the total axoplasmic Ca by up to 0-8 mm.…”

“…In the previous paper we examined the voltage sensitivity of that component of Na efflux which is coupled to Ca inflow through external Ca-internal Na exchange (Allen & Baker, 1986) and in this paper we extend this analysis to the two major components of Ca outflow. Unlike Ca inflow where all the major components respond to changes in membrane potential in the same general way, the two major components ofCa outflow respond oppositely.…”

Two light-dependent conductances in Lima rhabdomeric photoreceptors.

“…6). The voltage dependence of the Na/Ca exchanger, on the other hand, is quite modest, of the order of 8-32% for a 25-mV step in membrane potential (DiPolo et al, 1985; see also Allen and Baker, 1986). Finally, in the few cases where kinetic separation of the two waves appeared to be quite unambiguous (e.g., Fig.…”

Inactivation of outward Na(+)‐Ca2+ exchange current in guinea‐pig ventricular myocytes.

“…7C for intact myocytes, a decrease of the extracellular Ca2+ concentration from 1 to 0-1 mm did not reduce voltage dependence significantly in the presence of Li+. Neither a change of the extracellular Ca2' affinity per se (Allen & Baker, 1986a) Allen & Baker (1986b) reported that Ca2+-Ca2+ exchange was voltage independent in the absence of monovalent cations. Obviously, much further work is necessary to cla.rify how monovalent cations act and how these actions are related to the inherent electrogenicity of the Na+-Ca2+ exchange cycle.…”